ITS ONTOGENY, MORPHOLOGY, PHYLOGENY, AND FUNCTION. 409 



but continues laterad with it (Fig. 25, trt. tor. ell.), and then, bending ventrad from 

 the junction of the tectum and torus, follows for a distance parallel with the fibres 

 of the posterior commissure. This fibre tract is followed with great difficulty because 

 of its proximity to other fibre bundles, with which it is easily confused. I believe it, 

 however, to be identical with a fibre tract which has been described by several writers 

 as running from the anterior portion of the tectum to the cerebellum. The cells 

 of origin of this tract have never been determined. Johnston (:01) has described 

 such a tract in Acipenser as the tractus tecto-cerebellaris. It " courses obliquely around 

 the lateral border of the tectum" and enters the cerebellum. Moreover, I have in 

 other groups (selachians and birds) traced completely, and elsewhere described 

 (Sargent, : 01 a , : 04), the homologue of this tract- Sala showed in his figures the incom- 

 pletely impregnated fibres which form this tract, running a short distance into the 

 tectum, but he did not distinguish them from the fibres of the toro-tectal tract. 



5. Tractus Toro-fibrae Reissneris.— The chief centrifugal neurite, or axon, of the 

 torus cell is a little coarser than the neurites previously described. The axons of 

 the torus cells may, in general, be said to have a cephalo-ventral direction. In young 

 larval stages of many teleost's the axons of the ventral and median cells of the torus 

 enter the ventricle directly from the ventral surface of the torus, and particularly 

 in the median sulcus dividing the two lobes. But in older and adult stages the axons 

 are aggregated into more definite tracts which enter the ventricle by one of two paths. 

 The axons from cells in the ventral, lateral, and posterior portion of the torus 

 lobes form the tractus toro-fibrae Reissneris posterior which runs forward through 

 the torus lobes to a position above the posterior commissure where it curves ventrad 

 and caudad and, joining the corresponding tract from the opposite side, enters 

 the recessus of the mesoccele above the posterior commissure (Figs. 24, 25, trt. 

 fbr. Reis. p.). This tract usually enters the recessus in several fasciculi of varying 

 sizes formed by the fusion of the axons, and in which the separate axons cannot be 

 distinguished. 



The axons coming from the cells of the torus which are located in the dorsal, 

 median, and anterior portions of the torus lobes form the tractus toro-fibrae Reissneris 

 anterior (Figs. 20, 24, 25, trt. fbr. Reis. a.). These tracts run cephalad through the 

 dorsal portion of the torus lobes on either side the median plane and anterior to the 

 posterior commissure; they converge in the median plane and enter the ventricle 

 in compact fasciculi in which the constituent axons can be recognized only in very 

 early stages of development. Caudad of the posterior commissure all these separate 

 branches unite to form the fibre of Reissner (Fig. 24, fbr. Reis.) which runs posterior 

 into the canalis centralis and through the posterior portion of its course gives off 



